The present invention relates to a turbo decoder and method of implementing the same, which applies a base-2 binary LogMAP algorithm in implementing a turbo decoder used as an error correction code in wireless communication systems such as IMT-2000 (International Mobile Telecommunication-2000), to thereby reduce the hardware requirement and enhance the operation speed thereof as against a conventional turbo decoder based on LogMAP algorithm. The present invention also relates to a computer-readable medium storing a program for implementing the above method therein.
Recently, in the next generation mobile communication system such as IMT-2000, a turbo code is being employed as an error correction code for high-speed data transmission and a number of studies are under way on such turbo code.
An error correction approach using the turbo code has been proposed by Claude Berrou (1993), which became the center of attention in error correction applications.
It is known that the turbo code-based error correction technique has an enhanced bit error rate (BER) performance in proportional to iteration times, and has the ability to transmit information without invoking any error up to Shannon""s channel capacity. C. Berrou presents a schematic structure of the turbo decoder in its reference.
In general, an algorithm for implementing the turbo decoder may be divided into two approaches: one is MAP (Maximum A Posterior) algorithm and the other is SOVA (Soft Output Viterbi Algorithm) algorithm.
The SOVA algorithm, unlike the MAP algorithm, has advantages that it minimizes a sequence error rate and decreases an overall computational complexity of the turbo decoder, thereby making it easier to implement the turbo decoder. But, the SOVA algorithm has a disadvantage that it has its bit error rate lower than that of the MAP algorithm.
On the one side, the MAP algorithm has advantages that it minimizes the bit error rate and offers a significant performance of above 0.5 dB. Unfortunately, it has an increased computational complexity thereby making it difficult to implement the turbo decoder and hence it is susceptible to parameters so that the performance of the turbo decoder is extremely vulnerable to the parameters.
Thus, a number of studies are under way on a MAP algorithm with an enhanced performance. As an example, there is disclosed a structure of turbo decoder configured by a Modified MAP algorithm by S. S. Pietrobon. Further, a turbo decoder implemented with a Field Programmable Gate Array (FPGA) has been published. In the reference proposed by S. S. Pietrobon, a conventional LogMAP algorithm is simplified to induce a base-e function and a state metric and log-likelihood ratio (LLR) are calculated using the base-e function. However, it is difficult to implement the base-e function in hardware to thereby entail an increased hardware requirement. To overcome the foregoing problems, MaxLogMAP and SubLogMAP approaches, which approximate the base-e function and operate in a logarithmic region, have been published. As an example, MAX* algorithm has been published by W. J. Gross, wherein the base-e function is modified to Jacobi logarithm which is simplified in two steps. However, such an approximation results in a degraded performance of the turbo decoder.
As described above, in implementing the conventional turbo decoder, although the LogMAP algorithm, the SubLogMAP algorithm, the MaxLogMAP algorithm and the SOVA algorithm are being mainly used, it is required to reduce the hardware requirement and obtain the greatest possible performance in the aspect of hardware implement such as ASIC or FPGA. Unfortunately, the conventional LogMAP algorithm, which utilizes the base-e function to implement the turbo decoder, has a disadvantage that it is difficult to implement the base-e function in hardware. In addition, the conventional SubLogMAP and MaxLogMAP approaches, although easy to implement the turbo decoder, has severe disadvantage with regard to the performance.
Accordingly, what is needed is an approach to reduce the hardware requirement without invoking a degraded performance, in implementing the turbo decoder.
It is, therefore, an object of the present invention to provide a turbo decoder and method of implementing same, which applies a base-2 binary LogMAP algorithm in implementing a turbo decoder used in an error correction code in wireless communication systems such as IMT-2000, to thereby reduce the hardware requirement and implement a high-speed turbo decoder as against a conventional turbo decoder based on LogMAP algorithm.
It is another object of the present invention to provide a computer-readable medium storing a program for implementing the above method therein.
In accordance with one aspect of the present invention, there is provided a turbo decoder used as an error correction code in wireless communication systems, wherein a binary LogMAP algorithm is used, which comprising: means for splitting the sum of two input state metrics into an integral and a decimal part; means for comparing the integral parts of the two state metrics with one another to extract a maximum and a minimum integer value therefrom; means for obtaining a difference between the original integral part and the maximum or minimum integer value; means for calculating the sum of exponential terms of base-2 function in the decimal parts of the two input state metrics split by the splitting means; means for shifting only a decimal part with a smaller integral part among the exponential sums by the difference; a first addition means for adding the decimal part shift by the shifting means and a decimal part with a larger integral part among the exponential sums; means for applying a base-2 logarithm on the decimal part added by the adding means to thereby obtain a final value for the decimal part; and a second addition means for adding the maximum integral value from the comparing means and the final value for the decimal part from the applying means to thereby obtain a final value of the base-2 function.
In accordance with another aspect of the present invention, there is provided a turbo decoder used as an error correction code in wireless communication systems, wherein a binary LogMAP algorithm is used, which comprising: means for splitting the sum of each input state metrics into an integral and a decimal part; means for comparing the integral parts of said each state metrics with one another to extract a maximum and a minimum integer value therefrom; a first subtraction means for calculating a difference between the original integral part and the maximum or minimum integer value; means for calculating the sum of exponential terms of base-2 function in the decimal parts of said each input state metrics split by the splitting means; means for shifting the sum of exponential terms of the decimal parts by the difference; a first addition means for adding the sums of exponential terms of the decimal parts shift by the shifting means; means for applying a base-2 logarithm on the decimal parts added by the first addition means to thereby obtain a final value for the decimal part; a second addition means for adding the maximum integral value from the comparing means and the final value for the decimal part from the applying means to thereby obtain a final value of the base-2 function; and a second subtraction means for subtracting likelihood values according to the final value of base-2 function to thereby obtain a log-likelihood ratio (LLR).
In accordance with yet another aspect of the present invention, there is provided a method of implementing a turbo decoder used as an error correction code in wireless communication systems, wherein a binary LogMAP algorithm is used, which comprising the steps of: (a) splitting the sum of two input state metrics into an integral and a decimal part; (b) comparing the integral parts of the two state metrics with one another to extract a maximum and a minimum integer value therefrom; (c) obtaining a difference between the original integral part and the maximum or minimum integer value; (d) retrieving the sum of exponential terms of base-2 function in the decimal parts of the two input state metrics, and shifting only a decimal part with a smaller integral part among the exponential sums by the difference; (e) adding the shifted decimal part and a decimal part with a larger integral part among the exponential sums, and applying a base-2 logarithm on the added decimal part to thereby obtain a final value for the decimal part; and (f) adding the maximum integral value and the final value for the decimal part to thereby obtain a final value of the base-2 function.
In accordance with yet another aspect of the present invention, there is provided a method of implementing a turbo decoder used as an error correction code in wireless communication systems, wherein a binary LogMAP algorithm is used, which comprising: (a) splitting the sum of each input state metrics into an integral and a decimal part; (b) comparing the integral parts of said each state metrics with one another to extract a maximum and a minimum integer value therefrom; (c) calculating a difference between the original integral part and the maximum or minimum integer value; (d) calculating the sum of exponential terms of base-2 function in the decimal parts of said each input state metrics, and shifting the sum of exponential terms of the decimal parts by the difference; (e) adding the sums of exponential terms of the shifted decimal parts, and applying a base-2 logarithm on the added decimal parts to thereby obtain a final value for the decimal part; (f) adding the maximum integral value and the final value for the decimal part to thereby obtain a final value of the base-2 function; and (g) subtracting likelihood values according to the final value of base-2 function to thereby obtain a log-likelihood ratio (LLR).
In accordance with yet another aspect of the present invention, there is provided a computer-readable medium, in a turbo decoder incorporating a processor therein, storing a program for implementing the following functions of: splitting the sum of two input state metrics into an integral and a decimal part; comparing the integral parts of the two state metrics with one another to extract a maximum and a minimum integer value therefrom; obtaining a difference between the original integral part and the maximum or minimum integer value; calculating the sum of exponential terms of base-2 function in the decimal parts of the two input state metrics, and shifting only a decimal part with a smaller integral part among the exponential sums by the difference; adding the shifted decimal part and a decimal part with a larger integral part among the exponential sums, and applying a base-2 logarithm on the added decimal part to thereby obtain a final value for the decimal part; and adding the maximum integral value and the final value for the decimal part to thereby obtain a final value of the base-2 function, wherein a binary LogMAP algorithm is used.
In accordance with yet another aspect of the present invention, there is provided a computer-readable medium, in a turbo decoder incorporating a processor therein, storing a program for implementing the following functions of: splitting the sum of each input state metrics into an integral and a decimal part; comparing the integral parts of said each state metrics with one another to extract a maximum and a minimum integer value therefrom; calculating a difference between the original integral part and the maximum or minimum integer value; retrieving the sum of exponential terms of base-2 function in the decimal parts of said each input state metrics, and shifting the sum of exponential terms of the decimal parts by the difference; adding the sums of exponential terms of the shifted decimal parts, and applying a base-2 logarithm on the added decimal parts to thereby obtain a final value for the decimal part; adding the maximum integral value and the final value for the decimal part to thereby obtain a final value of the base-2 function; and subtracting likelihood values according to the final value of base-2 function to thereby obtain a log-likelihood ratio (LLR), wherein a binary LogMAP algorithm is used.